In the defect detection of sample surface structure, there is an increasing need for spectroscopic measurement using a broad range of wavelengths in the highly sensitive DUV-UV region (from 190 to 400 nm). At this time, lens systems must be color corrected when a broad range of wavelengths and a plurality of wavelengths are used. There are two methods of color correction, one using a reflective optical system and the other using a refractive optical system.
As an example of the reflective optical system, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-127830 (Patent Literature 1) describes a spectroscopic optical system provided with a color corrected lens system of Schwarzschild type for ultraviolet region.
As an example of the refractive and diffractive optical system, JP-A No. 2008-90051 (Patent Literature 2) describes an optical system that is color corrected with respect to an ultraviolet wavelength λ and a wavelength 2λ that is twice the wavelength λ, using diffractive optical element.
As an example of the refractive optical system, Japanese Patent No. 3288441 (Patent Literature 3) describes near-ultraviolet objective lenses that are color corrected for wavelengths over 350 nm in the near ultraviolet to visible region to have the same focal positions, allowing high resolution observation with near ultraviolet light as well as ultraviolet-fluorescent confocal imaging. Further, JP-A No. Sho 61-90115 (Patent Literature 4) describes image-forming objective lenses using fluorite and quartz as lens materials, which are color corrected in a wide range from the ultraviolet at a wavelength of about 200 nm to infrared region.